I hate to be bearer of bad news, but windows XP only supports up to 4gb of RAM, but even with that, it has a lot of bugs, and most programs only recognize 2 or 3 GB. Unless of course you have the 64bit Windoes XP, but that is ridden with bugs and has virtually no driver support. Not only that, but if you really got a server motherboard, then most likely it has a socket 940 (assuming you have AMD), in which case, you would have to have an opteron CPU, which requires you to have ECC RAM, which is more expensive than normal RAM, and is slower to boot, because it's designed to be very stable for servers that are unattended for years on end. And like everybody else said, the more unallocated RAM you have, the slower the computer gets, also not mentioning that it would have very high timings, which makes it slower.
so, umm, err yeah